<!DOCTYPE html>
<html lang="zh-CN">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        #first {
            width: 200px;
            height: 200px;
            border: 1px solid red;

        }

        #second {
            width: 200px;
            height: 200px;
            border: 1px solid blue;
            float: left;
        }
    </style>
    <script>
        alert("会出现什么呢")
    </script>
    <script src="my.js"></script>
</head>

<body>
    <div id="first">
        第一个盒子
    </div>
    <div id="second">
        第二个盒子
    </div>
    <div class="third"></div>
    <div class="fourth"></div>
    <div class="fifth"></div>

    <input type="button" value="唐伯虎" id="tbh">
    <div id="div"> 123</div>
    <script>
        var btn = document.getElementById('div')
        btn.onmouseenter = function () {
            alert('点秋香')
        }
    </script>
</body>

</html>